Non-linear,voltage variable electrical resistor



United States Patent US. Cl. 252-516 7 Claims ABSTRACT OF THE DISCLOSURE A non-linear, voltage variable electrical resistor comprising a body of particles of silicon carbide in a vitrified glass binder. The glass binder being in the amount of between 2% and 60% by volume and, preferably, between 10% and 40% by volume.

BACKGROUND The present invention relates to a novel composition of a non-linear, voltage variable electrical resistor, generally called a varistor. Heretofore, such resistors comprised a body of particles of silicon carbide in a matrix of a crystalline ceramic binder. The body may also contain such additional elements as a clay filler or graphite to improve certain electrical characteristics of the resistor. These resistors are made by mixing together the various ingredients which are all in the form of particles. The mixture is then formed into a desired shaped body either by extrusion or pressing. If the body is formed by extrusion, an extruding agent, such as colloidal magnesium, aluminum silicate, can be included in the mixture. The body is then fired to sinter the particles of the ceramic binder together. One problem in making such resistors is that they are fired at relatively high temperatures between 1100 C. to 1300 C., and the length of the firing cycle is commonly several hours. This long firing time adds to the manufacturing cost of the resistors. Another problem arises in obtaining a low E voltage characteristic, where E is the voltage necessary to obtain one milliampere of current. To obtain low E s it has been the practice to either increase the graphite content of the resistor composition or to make the resistor body thinner. However, both of these methods result in a physically weaker body.

SUMMARY OF INVENTION It is an object of the present invention to provide a novel composition of a non-linear, voltage variable electrical resistor.

It is another object of the present invention to provide a novel composition of a non-linear, voltage variable electrical resistor which is fired at relatively low temperatures for a short firing cycle so as to decrease the time and cost for making the resistor.

It is still another object of the present invention to provide a novel composition of a non-linear, voltage variable resistor which provides a resistor having a relatively low E voltage characteristic and high physical strength.

It is a further object of the present invention to provide a non-linear voltage variable resistor which uses vitrified glass as the binder for the silicon carbide.

Other objects will appear hereinafter.

The invention accordingly comprises a composition of matter and a product made therefrom possessing the 3,509,072 Patented Apr. 28, 1970 characteristics, properties, and the relation of constituents which will be exemplified in the following description, and the scope of the invention will be indicated in the claims.

DESCRIPTION OF INVENTION The non-linear, voltage variable electrical resistance material of the present invention comprises a mixture of particles of silicon carbide and a vitrified glass frit binder. The proportion of the glass frit in the mixture is between 2% and 60% by volume and, preferably, between 10% and 40% by volume. In addition, the resistance material may include a filler, such as clay, and small amounts of graphite to lower the rated operating voltage. Also, if the material is to be extruded to form the resistor body, an extruding agent, such as colloidal magnesium aluminum silicate may be included or, if the material is to be applied as a coating, an organic coating vehicle, such as butyl Carbitol acetate may be included.

The vitrified glass frit used in the resistance material of the present invention may be of any well-known composition which has a softening point between C. and 1500 C. and preferably between 350 C. and 1000 C. Preferably, the glass should be a non-alkali glass to avoid ionic conduction in the resistor body and should have a thermal expansion similar to that of silicon carbide. Also, it is desirable that the glass have a low solubility in water and be resistant to chemical attack. One type of glass frit which has been found to be satisfactory for the resistor of the present invention are the borosilicate frits, such as bismuth, cadmium, barium, calcium, aluminum or other alkaline earth borosilicate frits. The preparation of such glass frits is well known and consists, for example, in melting together the constituents of the glass in the form of the oxide of the constituents and pouring such molten composition into water to form the frit. The batch ingredients may, of course, be any compound that will yield the desired oxides under the usual condition of frit production. For example, boric oxide will be obtained from boric acid, silicon dioxide will be produced from fiint, barium oxide will be produced from barium carbonate, etc. The coarse frit is preferably milled in a ball mill with water to reduce the particle size of the frit and to obtain a frit of substantially uniform size. The particle size of the glass frit should be less than 50 mesh and preferably smaller than 325 mesh.

To make the non-linear, voltage variable resistance material of the present invention, the glass frit, silicon carbide particles and any additional ingredient, such as a clay filler, graphite and colloidal magnesium, aluminum silicate are thoroughly mixed together. If the resistance material is to be formed into a resistor body by extruding or pressing, the ingredients are mixed together in a muller with sufficient water to give the mixture the desired consistency for the particular forming process. If the resistance material is to be coated on a substrate, the ingredients may be mixed together by ball milling in water or an organic medium, such as butyl Carbitol acetate or a mixture of butyl Carbitol acetate and toluol.

To make a resistor with the resistance material of the present invention, the resistance material is formed into a body of the desired shape and size. The body may be formed by extruding the material under pressure through a die into rods or tubes, or by pressing the material under pressure into disk shaped bodies, or by coating the material on the surface of a substrate. The substrate may be a body of an electrical insulating material, such as a ceramic, or a metal body which will act as one terminal of the resistor. The resistance material may be coated on the substrate by brushing, dipping, spraying or screen stencil application. The resistance body is then fired in a conventional furnace at a temperature at which the glass frit softens. Although the resistor body may be fired in air, it is preferable to fire the body in an inert atmosphere such as argon, helium or nitrogen, or a reducing atmosphere, such as hydrogen or a mixture of nitrogen and hydrogen. When the resistance body is cooled, the glass hardens to provide a body of vitreous glass having the silicon carbide embedded therein and dispersed therethroughout. If the body is formed as long rods or tubes or formed as a coating on long substrates, the body is then cut into shorter pieces of a length to provide the desired resistance value. Terminals are then attached to the body and the body may be provided with a protective coating or housing.

The following examples are given to illustrate certain preferred details of the invention, it being understood that the details of the examples are not to be taken as in any way limiting the invention thereto.

EXAMPLE I A resistance material of the present invention was made of, by volume, 60%-100 grit silicon carbide particles, glass frit and 30% kaolin filler. The glass frit was a magnesium, aluminum, borosilicate glass. The resistance material was pressed into disks which were one inch in diameter and about 0.10 inch thick. The disks were fired in air over a 30 minute cycle having a peak temperature of 1000 C. The resulting non-linear resistors had an E i.e. voltage necessary to obtain one milliampere of current, of 163 volts and a voltage ratio of 1.62, the vo tage ratio being calculated by dividing the voltage necessary to obtain one milliampere of current by the voltage required for one-tenth milliampere of current.

EXAMPLE II A resistance material of the present invention was made by mixing together the following ingredients in the proportions indicated by volume:

Percent Silicon carbide100 grit 60 Glass frit (magnesium, aluminum borosilicate glass) 5 Kaolin 35 The resistance material was pressed into disks which were one inch in diameter and about 0.10 inch thick. The disks were fired in a nitrogen atmosphere over a thirty minute cycle having a peak temperature of 1000" C. The resulting resistors had an E of 96 volts and a voltage ratio of 1.76.

EXAMPLE IV Resistance materials of the present invention were made by mixing together the ingredients shown in the following table in the proportions indicated in percent by volume:

Silicon carbide (100 grit) 50 50 50 Glass frit (magnesium, aluminum 0 cate glass) 10 20 30 Oalcined kaolin 29. 5 19. 5 9. 5

Colloidal magnesium aluminum silicate..- 7. 5 7. 5 7. 5

Graphite 3 3 3 The mixtures were extruded into tubes which were fired in a nitrogen atmosphere over a thirty minute cycle at a peak temperature of 900 C. The fired tubes were cut into bodies approximately 0.6 inch in length and having an outside diameter of approximately 0.15 inch and an inside diameter of approximately 0.07 inch. The resulting resistors had the characteristics shown in the following table.

Mechanical strength (p.s.i.)

lm (volts) Voltage ratio EXAMPLE V Resistors were made from the three resistance materials of the present invention described in Example IV by extruding the materials into tubes which were fired in a nitrogen atmosphere over a thirty minute cycle at a peak temperature of 925 C. The fired tubes were cut into bodies of the same size as described in Example IV. The resulting resistors had the characteristics shown in the following table:

Mechanical strength (p.s.i.)

E (volts) Voltage ratio EXAMPLE VI Resistors were made from the three resistance materials of the present invention described in Example IV by extruding the materials into tubes which were fired in a nitrogen atmosphere over a one hour cycle at a peak temperature of 925 C. The fired tubes were cut into bodies of the same size as described in Example IV. The resulting resistors had the characteristics shown in the following table:

Mechanical strength (p.s.i.)

E1 .0 (volts) Voltage ratio of said mixture being fused to each other by sintering at a' predetermined peak temperature which is substantially only sufiicient to soften the glass frit.

2. A non-linear, voltage variable electrical resistance material in accordance with claim 1 in which the amount of the glass frit is between 10% and 40% by volume.

3. A non-linear, voltage variable electrical resistance material in accordance with claim 1 in which the glass frit is a glass having a softening point between C. and 1500" C.

4. A non-linear, voltage variable electrical resistance material in accordance with claim 3 in which the glass frit is a non-alkali glass.

5. A non-linear, voltage variable electrical resistance material in accordance with claim 4 in which the glass frit is of a glass having a coefiicient of thermal expansion substantially equal to the coefiicient of thermal expansion of silicon carbide.

6. A non-linear, voltage variable electrical resistor comprising a self-supporting body of fused vitrified glass and particles of silicon carbide embedded in and dispersed throughout the glass with the amount of glass being between 2% and 60% by volume, said particles of silicon carbide being fused to a frit of said glass by sintering at a predetermined peak temperature which is substantially only suflicient to soften the glass frit.

7. A non-linear, voltage variable electrical resistor in accordance with claim 6 in which the amount of glass forming the body is between 10% and 40% by volume.

References Cited UNITED STATES PATENTS 1,012,531 12/1911 Egly 10644 2,331,852 10/1943 Stoelting 33821 Ferguson et a1. 33821 Remington et a1. 2525 16 X Janakirama-Rao 252516 X Masuyama et a1. 33820 X US. Cl. X.R. 

